Q on brake balance, opinions wanted

[Viperzilla]

I just wanted to post a quick question on brake balance. In your own opinion, does the rear lock up more prematurely than you'd like, or doesn't stop fast enough? Any other brake suggestions that go along with this topic would be appreciated. Please don't post recommendations for any companies. Thanks in advance for the info!
Sean

 

[Stephen Yap]

Rears on pre SRT-10 models need more bias. It is difficult to lock them up prematurely or at all (no handbrakes allowed) because of how the way the front to rear brake bias was engineered. An aftermarket MC will help balance some of it out. I am curious as to what the differences are between the GenI/II MC are vs. the SRT-10 MC. It can't be just the improved ABS, calipers, and wider rubber ???

 

[Viperzilla]

Rubber is the same width, I believe on Gen I,II, and III. If not, the SRT has a +10 mm tread width, which isn't that much.

 

[Bugeater]

Why do you have to upgrade the MC? Why cant you remove the 43% proportioning valve instead? Wont this equalise or at least better rear bias by a net 14%?

 

[CAS]

It's definitly not just improved rubber, bigger rotors/calipers, or improved bias; the improved suspension geometry of the SRT allows more braking force to be applied to all four wheels and for that force to really compound the area the contact patch(s).

 

[Stephen Yap]

Bugeater, yes that would work though I don't like those bicycle calipers in the rears

Clint, do you know exactly what was done to the suspension geometry and why it works given that the weight distribution remains relatively the same on the Gen IIs ??

 

[Tom F&L GoR]

The front brakes are decent 4-piston Brembos, the rears are single piston calipers from a Renault Alliance/Dodge Monaco vintage late 1980's. You don't get enough brake torque from the rears, so the fronts lock up first and easily.

You can disable the OEM proportioning valve (do a search, lots of posts) which gets rid of the rear brake line pressure limiting effect and allows full brake pressure to rears under any circumstance. When I did this, I still could not lock up the rears (first.)

An unnamed brake company with many Viper followers (you said no names) has a Stage 1 caliper that reduces the front brake piston sizes, so as to better balance the front and rear contribution. Or if you work at the other end, increase the rear caliper piston from 36mm to 38mm and you'll get similar balance results. In my opinion, you would still not lock up the rears first even with removing the brake line pressure limiting feature.

There are some that are going to a 40mm rear brake piston size, I personally find it only ever so slightly over the edge of balance, so really should install a proportioning valve. However, the appearance is stock, so no special fabricating of the upright, nor special pads are needed. I've melted a few rear wheel tire valve caps, when parked above the caliper, so I need to get more rear brake cooling.

I would estimate that the pressure limiting valve removal makes it about 20% better, the 38mm size caliper in the rear is another 40%, and the 40mm rear is 100% balanced-for me, anyway. Balance does not mean best heat rejection, it's actually probably worse, since all four rotors are now efficiently absorbing lots of heat. All those big buck race calipers and rotors can take more heat and provide balance; this system provides balance and OEM level heat rejection.

Changing to another master cylinder won't change front to rear balance, since the MC bore is straight through and not staggered. Both ends will get the same pressure.

The SRT-10 looks to have 4-piston calipers front and rear; therefore with more than enough capacity at the rear, it can be throttled down to the correct level with a proportioning valve.

 

[kverges]

Tom said it well. Bottom line is pre-ABS Vipers don't have enough rear bias.

Avoid the knee-**** reaction of bigger calipers on front; smaller pistons in the front is the only way to go unless you change the rear.

Tom is too humble, too. He engineered a slick way to increase the stock caliper bores size in the rear that I have to say is very neat and stealthy. Except for heat issues, this is the way to go. To deal with heat, you will have to go with larger rotors and/or ducting. It is all available here from modest items like Porche air deflectors to more radical ducting solutinos and full-on aftermarket calipers and rotors.

The ABS cars have near perfect front-rear balance from the factory, due to larger rear calipers (don't try to upgrade, as you will need new rear uprights and 18" wheels at the very least, not to mention the ABS pump, controller, tone rings, sensors and wiring). That is the best kept secret about the '01-'02 Viper is that the brakes are not just an "add-on" ABS set up, but with better balance, too.

 

[Stephen Yap]

"Changing to another master cylinder won't change front to rear balance, since the MC bore is straight through and not staggered. Both ends will get the same pressure."
***********************************************************

Tom's right, I forgot to add that you need an aftermarket prop valve to then adjust bias.

 

[CAS]

Stephen,

I apologize for not seeing your post more quickly. I have, after some research, not been able to track down exactly what was done to the SRT to improve it's suspension geometry. The exact post from which I made my comments I cannot find . I suspect the car has revised bump/rebound settings, camber/caster changes, and perhaps the swing arms and shock postions have been optimized for more progressive distribution of weight and better placement of Cg..

Hope this helps, I do apologize for the speculation as I thought I could easily pull up where I read that.

Regards,
Clint

 

[Tom F&L GoR]

Yup, the ABS cars have a 43mm rear piston size, so "more" than needed, but brake balance is then handled by the ABS system. And as Keith said, it won't fit on a Gen 1 - I know from having the parts in front of me and seeing it. The caliper body is larger, so a 17" wheel won't even clear it and the attaching bracket system is completely different than a Gen 1/2 caliper.

I drove the yellow SRT at the Englishtown AX event; while I like my car's setup better overall, the ABS on the '03 was a surprisingly excellent system. I've never driven an ABS car (like I stole it) before and it's truly a great aid in saving tires, turning while braking, staying in control no matter how stupid I was... really neat.

And Stephen, a larger MC still won't work, even with the prop valve. The problem is that you need more pressure to the rear than the front, and prop valves only reduce pressure the rear, not increase. A larger MC will increase pressure to the front, where you don't need it.

 

[Stephen Yap]

Clint, no problem. Post the info when you find it.

Tom, a larger MC will put more pressure to both front and rear. Put a prop valve to decrease the increased pressure to the front basically getting it to match the unchanged piston sizes. A second prop valve for the rear turned up to allow for more pressure to the rear or to match any change in piston sizes there. Seems to work on my car.

 

[Bugeater]

So when did the rear piston diameter change from 36mm to 38mm to 43mm?

I didnt know 2001's and up got 43mm rears!

Has anyone tried Sean Roe's rear brake upgrade? Not sure who the manufacturer is, but its a 2 piston setup. I am trying to reach better balance with my Stoptech Fronts without upgrading to a 4wheel system. The upgrade path is not very efficient pricewise.

 

[Janni]

Based on some issues we had at Gingerman, you could probably play around with pad compounds and get more rear brake. It's not for the faint of heart, as you can swap ends fairly quickly and rear lockup is a b*tch when you blow out BOTH back tires.... but you can definitely get more rear brake....

Had we kept both of our 1996's, we would have done Tom's rear brake mod - it's a VERY well engineered solution that addressed the problem at the root cause.....

I agree that the 2001/2002 dynamic brake proportioning was one of the best kept secrets of the later model vipers - it was odd riding in a Viper that would stop without the nose diving to the ground.....